The equation of motion for a "hard" spring is
$$
\ddot{x}(t)+\left(1+a^2 x(t)^2\right) x(t)=0 .
$$
Analytically determine the period and the solution $x(t)$ for the hard spring, given the initial conditions $x(0)=A, \dot{x}(0)=0$. Plot the solution over several cycles for $a=A=1$.